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  1. #include <iostream>
  2. #include <cstddef>
  3. #include <utility>
  4. #include <cassert>
  5. #include <array>
  6.  
  7. 	template <typename first, typename ... rest>
  8. 	struct find_biggest_size
  9. 	{
  10. 		constexpr static std::size_t value = (sizeof(first) > find_biggest_size<rest...>::value) ? sizeof(first) : find_biggest_size<rest...>::value;
  11. 	};
  12.  
  13. 	template <typename first>
  14. 	struct find_biggest_size<first>
  15. 	{
  16. 		constexpr static std::size_t value = sizeof(first);
  17. 	};
  18.  
  19. 	template <typename first, typename second>
  20. 	struct find_biggest_size<first, second>
  21. 	{
  22. 		constexpr static std::size_t value = (sizeof(first) > sizeof(second)) ? sizeof(first) : sizeof(second);
  23. 	};
  24.  
  25. 	template <typename type, typename first, typename ... rest>
  26. 	struct is_present
  27. 	{
  28. 		static constexpr bool value = (is_present<type, rest...>::value) ? true : std::is_same<type, first>::value;
  29. 		static constexpr std::size_t index = is_present<type, rest...>::index + 1;
  30. 	};
  31.  
  32. 	template <typename type, typename first>
  33. 	struct is_present<type, first>
  34. 	{
  35. 		static constexpr bool value = std::is_same<type, first>::value;
  36. 		static constexpr std::size_t index = 0;
  37. 	};
  38.  
  39. 	template <std::size_t ind, typename ... Types>
  40. 	struct extract
  41. 	{};
  42.  
  43. 	template <std::size_t ind, typename first, typename ... rest>
  44. 	struct extract<ind, first, rest...>
  45. 	{
  46. 		using type = typename extract<ind - 1, rest...>::type;
  47. 	};
  48.  
  49. 	template <typename T>
  50. 	struct extract<0, T>
  51. 	{
  52. 		using type = T;
  53. 	};
  54.  
  55. 	template <typename T>
  56. 	struct in_place_t
  57. 	{};
  58.  
  59. 	struct bad_variant_access
  60. 	{};
  61.  
  62. 	template <typename ... alternatives>
  63. 	class variant
  64. 	{
  65. 		char object[find_biggest_size<alternatives...>::value];
  66. 		std::size_t index;
  67. 	public:
  68. 		template <typename T>
  69. 		variant(T&& other)
  70. 		{
  71. 			static_assert(is_present<T, alternatives...>::value, "type is not in range");
  72. 			new ((T*)&object[0]) T(std::forward<T>(other));
  73. 			index = is_present<T, alternatives...>::index;
  74. 		}
  75.  
  76. 		template <typename T>
  77. 		variant(const T& other)
  78. 		{
  79. 			static_assert(is_present<T, alternatives...>::value, "type is not in range");
  80. 			new ((T*)&object[0]) T(std::forward<T>(other));
  81. 			index = is_present<T, alternatives...>::index;
  82. 		}
  83.  
  84. 		template <typename T, typename ... ArgTypes>
  85. 		variant(in_place_t<T>, ArgTypes&& ... args)
  86. 		{
  87. 			static_assert(is_present<T, alternatives...>::value, "type is not in range");
  88. 			new ((T*)&object[0]) T(std::forward<ArgTypes>(args)...);
  89. 			index = is_present<T, alternatives...>::index;
  90. 		}
  91.  
  92.  
  93.  
  94. 		template <std::size_t ind, typename ... alts>
  95. 		friend typename extract<ind, alts...>::type get(variant<alts...>& v)
  96. 		{
  97. 			using T = typename extract<ind, alts...>::type;
  98. 			if (v.index != ind)
  99. 			{
  100. 				throw bad_variant_access{};
  101. 			}
  102.  
  103. 			return *((T*)(v.object));
  104. 		}
  105.  
  106. 		~variant()
  107. 		{
  108. 			static std::array<void(*)(char*), sizeof...(alternatives)> destructors
  109. 			{
  110. 				std::addressof(invoke_destructor_impl<alternatives>)...
  111. 			};
  112. 			destructors[index](object);
  113. 		}
  114.  
  115. 	private:
  116. 		template<class T> static void invoke_destructor_impl(char* object)
  117. 		{
  118. 			auto pt = reinterpret_cast<T*>(object);
  119. 			pt->~T();
  120. 		}
  121. 	};
  122.  
  123. int main() {
  124. 		variant<int, char, bool> v(true);
  125. 		std::cout << get<2>(v);
  126.  
  127.  
  128. 	return 0;
  129. }
Success #stdin #stdout 0s 3468KB
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https://ideone.com/MUXf06
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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